In the past, in the method of production of blast furnace coke, from the point of view of the increase of the charged bulk density and resultant improvement of coke strength and the shortening of the carbonization time and resultant improvement of coke productivity, the practice has been to dry the coking coal containing moisture of 8 to 12% or so to reduce the moisture content in the coking coal to 5 to 6% or so and further to 0%, then charging and carbonizing it in a coke oven.
For example, the precarbon method of drying the coking coal to a moisture content of 0% and preheating it to a peak temperature of 150 to 230° C. or so, then charging and carbonizing it in a coke oven is known (for example, see “Coke Notes”, The Fuel Society of Japan 1988, pg. 134).
According to this method, the coke productivity is improved by approximately 35% compared to when not preheating coal. Further, the coke strength and other aspects of the quality of the coke are improved. Due to this, the ratio of the non- or slightly-caking coal or other poor quality coal with poor caking ability in the mixed coal can be increased to approximately 25%.
However, if drying or preheating the coking coal to reduce the moisture content in the coking coal to 5% or less or further to near 0%, the problem arises of the fine-grained coal easily producing dust in the process of transport of the coal and at the time of charging into the coke oven.
As prior art for solving this dust producing problem of fine-grained coal, the method has been proposed of drying and preheating the coal, then classifying it and forming only the fine-grained coal of 0.5 mm and 0.3 mm causing the dust production into masses.
For example, the method is known of drying and classifying the coking coal, kneading only the recovered fine-grained coal or the fine-grained coal in which part of the coarse-grained coal is added plus tar etc. to obtain pseudo particles and thereby suppressing the production of dust due to the fine-grained coal in the dry coal (for example, see Japanese Patent Publication (A) No. (A) 8-239669).
However, in this method, if the drying of the coking coal causes the moisture content in the coking coal to drop, the strength of the pseudo particles will drop due to the drop in the adhered moisture and they will crumble during transport, so it is not possible to dry the coal to reduce the moisture content in the coal too much. As a result, the effect of improvement of the coal bulk density in the coke oven and improvement of coke strength due to the drying of the coal could not be sufficiently obtained.
Further, a method of production of coke has been proposed of crushing the coal, drying and heating the mixed coal comprised of fine grains of 3 mm or less in an amount of 85 to 95% and the balance of coarse grains of 10 mm or less, adding and mixing 3 to 8% of tar to all of the mixed coal at a temperature of 140° C., rolling it at a temperature of 120° C. to obtain briquettes, and carbonizing them in a coke oven (for example, see Japanese Patent Publication (A) No. 52-71504).
Further, a method of production of coke has been proposed of drying coal to a moisture content of 0 to 2.7%, classifying it, adding tar in an amount of 3 to 5% to only the recovered fine-grained coal of 0.3 mm or less at a temperature of 80° C. or less, agglomerating the result by a grooved roll to form briquettes, and carbonizing the result together with the balance of the mixed coal, that is, the coarse-grained coal, in a coke oven (for example, see Japanese Patent Publication (A) No. 9-3458).
The briquettes obtained by these methods all are increased in strength of the masses compared with the above pseudo particles, so the masses can be kept from crumbling during transport. Further, by forming the coal into briquettes, the distance between fine powder particles in the coal becomes small, so the adhesion between fine powder particles at the time of carbonization of the briquettes in a coke oven rises and the coke strength is improved.
However, even by these methods, if the ratio of non- or slightly-caking coal with a low caking ability within the mixed coal is raised, it became difficult to sufficiently secure the strength of the coke even by the method of carbonizing the briquettes in the coke oven.
Further, when adding tar to the dried coal or preheated coal and agglomerating it by rolling, if agglomerating at a high temperature, the volatile ingredients within the tar form a gas, the pressure of the gas inside the rolled briquettes increases, the agglomerating becomes difficult, the briquettes cracks, and other problems arise causing a drop in productivity and product yield.
In particular, when classifying dried coal or preheated coal, then adding tar to only the fine-grained coal and rolling it, compared to when rolling mixed coal containing coarse-grained coal, the occurrence of cracks due to the coarse-grained coal in the briquettes at the time of agglomerating is suppressed, but the gas produced inside the briquettes at the time of agglomerating has a hard time escaping, so the above problem due to the increase in internal pressure in the briquettes becomes remarkable.
For these reasons, when adding tar to dried coal or preheated coal, in particular fine-grained coal, and agglomerating it by rolling, it was necessary to roll it in a state with the temperature of the fine-grained coal reduced to less than 80° C.
On the other hand, coking coal can be supplied stably and cheaply in terms of a resource, but it is required to manufacture coke of high strength cheaply and with high productivity when mixing a large amount of non- or slightly-caking coal or other poor quality coal with a low caking ability into the mixed coal.
By using the above coal drying or precarbon method, the bulk density at the time of charging the coal into the coke oven increases, so it is possible to secure a predetermined coke strength even when mixing in a certain large amount of non- or slightly-caking coal or other poor quality coal with a low caking ability.
However, with these methods, to secure a predetermined coke strength, the ratio of the non- or slightly-caking coal etc. with a low caking ability mixed in the mixed coal was limited to at most 25%.
As technology for solving this problem, in recent years, the method of production of coke has been proposed of modifying the entire of mixed coal containing the large amount of non- or slightly-caking coal or other poor quality coal with a low caking ability by rapidly heating until softening and melting at about 350° C. or more, higher than the heating temperature of the precarbon method, rolling the coal in the semi-molten state with the caking ability while maintaining the temperature at 350° C. or more to form briquettes, then carbonizing them in a coke oven (for example, see Japanese Patent Publication (A) No. 07-118665).
However, with the method of rapid heating the entire amount of dried and preheated mixed coal by an air flow tower, the differences in particle size between the fine-grained coal and the coarse-grained coal causes differences in the heating temperatures at the coal particles. In particular, the fine-grained coal loses its caking ingredients due to overheating and therefore the caking ability of the non- or slightly-caking coal cannot be sufficiently improved.
Therefore, to solve this problem, the method or production of blast furnace coke has been proposed of drying and preheating non- or slightly-caking coal mixed into the mixed coal in an amount of 10 to 60% at a temperature of 50 to 350° C., classifying it into fine-grained coal of a particle size of 0.3 mm or less and coarse-grained coal of a particle size of over 0.3 mm, rapidly heating said fine-grained coal to a temperature range of the softening start temperature to the maximum fluidity temperature at a rate of temperature increase of 1×103 to 1×105° C./minute, then hot agglomerating it at a pressure of 5 to 1,000 kg/cm2 in the state held at that temperature range, then mixing in said coarse-grained coal of the non- or slightly-caking coal and carbonizing the mixture in a coke oven (for example, see Japanese Patent Publication (A) No. 08-209150 and Japanese Patent Publication (A) No. 09-048977).
However, there were the following problems when using these rapid heating methods for coal to rapidly heat the entire amount of non- or slightly-caking coal in the mixed coal or only the fine-grained coal from the softening start temperature of 350° C. or more to the maximum fluidity temperature and rolling the result in a semi-molten state while maintaining a high temperature of 350° C. or more.
That is, it becomes difficult to charge semi-molten state coal into a roll molding machine and becomes necessary to shape it while controlling the temperature so as to prevent the caking ingredients from escaping or being oxidized in a high temperature state.
Further, it has been known in the past that the fine-grained part after crushing coal contains a larger amount vitrinite ingredients and other caking ingredients compared to the coarse-grained part. Because of this, the amount of improvement of the caking ingredient of the fine-grained coal due to the rapid heating is smaller compared to the coarse-grained coal in the coal. Rather, when the fine-grained coal is heated to a high temperature state, the deterioration due to escape or oxidation of the caking ingredient when the fine-grained coal is heated to the high temperature state becomes larger than that of the coarse-grained coal.
Further, when using this method to rapidly heat and modify the non- or slightly-caking coal contained in a large amount in the mixed coal, it is necessary to separately heat treat the fine-grained coal and coarse-grained coal in the non- or slightly-caking coal by an air current tank etc., so the cost of the equipment is expensive and the operating conditions also become complicated.
Consequently, the conventional coal rapid heating method cannot be said to be sufficient as a method using mixed coal containing a large amount of non- or slightly-caking coal to produce high strength coke inexpensively while maintaining a high productivity.